Closed captions are a text version of the spoken part of a television, movie, or computer presentation. Closed captioning was developed to aid hearing-impaired people, but it is useful for a variety of situations. For example, captions can be read when audio can not be heard, either because of a noisy environment, such as an airport, or because of an environment that must be kept quiet, such as a hospital.
Closed captioning information is encoded within a video signal, in line 21 of the vertical blanking interval (VBI). The text only becomes visible with the use of a decoder, which may be built into a television set or available as a set top box. In general, an onscreen menu on newer televisions allows you to turn closed captioning on or off.
Most programs are captioned in advance of transmission, but the nature of some programs, such as live sports events, requires real time captioning. For real time captioning, a stenographer listens to the broadcast and types a shorthand version into a program that converts shorthand into captions and adds that data to the television signal.
The closed captioning standard is defined by the Electronic Industries Alliance in EIA-708-B, “Digital Television (DTV) Closed Captioning.” EIA-708-B makes it possible for users to select the size, color, and font of their captions and to select among multiple streams, choosing, for example, a particular language.
The typical use of closed captioning is a text box directly overlaid on the video. This is not preferable since the text box blocks a portion of the video. Further, the closed captioning normally provides a visual transcription of the dialog. Often it is difficult to determine who was speaking, or in other words, to whom does the visual transcription in the closed captioning text box apply.